You could just use all OEM seadoo and all you would need to do would be make an adaptor at your through hole for the bellows, you would need to run a doo pump or have your impeller modded to suit the...

I have had my GTX230 grate off, did not realise it had a nut on top of the threaded plate and spun the assembly breaking the small tab. I'll call it a plate as it is not an insert. It is a cast alloy...

I could not find a non vented cap for my 12 RXPX so had one made , it is the same cap that now fits my GTX230.
If you do make your own just ensure the material you use will will not react with fuel....